This invention relates to measuring of the seal or lamination strength between the same or different web materials, particularly thermoplastic materials that may be heat sealed or laminated together, by pulling apart the sealed strips of the webs and measuring the force required to do so. It further relates to testing of the hot tack properties of such materials. "Hot tack" is the expression for the strength of heat seals subsequent to heat seal formation but prior to cooling of the seal to ambient conditions at which it has achieved its full strength, typically referred to as "seal strength." Information on hot tack properties is of interest to manufacturers and users of basic polymers, plastic films and laminates.
Hot tack properties are of particular importance in respect to so-called "form and fill" machines which create from film a continuous stream of packages closed by film-to-film seals which are filled with a product while a bottom seal is being made on each package by heat seal dies that apply heat and pressure. Heat sealed closures are made very rapidly and it is important that the film create a sufficiently high tack, or adhesiveness, quite quickly as the seal cools after it has been made and the heat seal dies removed. This is so that the package can be quickly filled without destruction of the seal by the weight of the product. That in turn permits the heat seal dies to be quickly moved on to the next seal.
Although few are designed or adapted for determining hot tack properties, the many seal strength testers have been proposed in the past for measuring various cold adhesive and/or heat activated adhesive seals, some with automation of functions and/or control of various sealing and testing variables. Typically, as in the case of the laminate bond tester described in U.S. Pat. No. 3,580,065, free ends of laminated strips are pulled apart at a uniform rate since the velocity of delamination is an independent variable that must be maintain the same between tests in order for the parting force measured to be comparable. In some cases for testers utilized for testing full seal strength, such as in U.S. Pat. Nos. 3,850,033, 4,637,252 and 4,893,513, the force is measured and recorded over the time that delamination of the seal proceeds, i.e. as a function of time. The maximum force thus recorded is taken as the delamination force. Alternatively this data is integrated to arrive at the energy value representing the energy required to delaminate the entire seal area.
In the case of the tester described in U.S. Pat. No. 4,958,521, the free ends of the laminate are pulled linearly in opposed directions at the same speed such that the line of delamination of the test strip remains essentially at the same position. This permits a guide roller adjacent this position to maintain the test strip at a desired angle as it approaches the location of delamination.
Various testers, such as those of U.S. Pat. Nos. 3,548,652, 3,850,033 and 4,637,252, and DDR Patent Publication 142756 laid open Jul. 9, 1980, are equipped to automatically first create seals or laminations of test strips under controlled conditions and then pull them apart and measure the force required.
The testers of U.S. Pat. Nos. 3,580,065 and 5,144,845 and Russian Patent 1259 157 A describe adhesion strength test procedures in which the temperature of the sample to be tested can be adjusted to a desired value. In the case of the Russian patent, the sample strip is coiled around a heated roller so that a temperature gradient is established along the strip. The sample is then pulled apart as it is unwound, beginning from the outside end of the coil and the delamination force measured as delamination proceeds to the gradually hotter material toward the inside end of the coil. In this fashion the delamination force can be determined for the sample at different temperatures.
A few testers, such as the DTC Hot Tack tester sold by Topwave Instruments Oy and the tester described in DDR Patent Publication 142756, are designed specifically for testing heat seals and which first create such heat seals. In the DTC Hot Tack tester, a test strip of film is folded or lapped on itself and an area of the overlapped film is then subjected to heat and pressure by heat seal dies at prescribed temperature and time conditions to create a heat seal. The dies are then opened to release the film and either immediately or after a set delay time the two free ends of the overlapped strip are rapidly pulled apart to break the seal, while measuring the pulling force. The maximum force measured is then taken as the hot tack strength of the film for the particular heat seal temperature and delay time (if any) utilized. These testers suffer the limitation that only a single force measurement (the maximum force measured) is taken for each sample for the particular heat seal dwell time and delay period selected and thus multiple tests would be required to measure the rate of gain of strength as the seal cools. Also, the rapid speed of peel propagation employed may not be representative of the conditions found in actual package sealing operations, as in form and fill machines. Cooling of the seal in these testers take place in an uncontrolled fashion, which leads to further imprecision in the test results. Additionally, while operation of these testers is automated to some degree, they are cumbersome in operation and require hand manipulation of a test sample for its insertion between the heat seal dies, thus making the operation susceptible to human error, particularly with inexperienced operators.